The present invention relates to a mobile communication system such as a time-division multiplexing access (as will be shortly called the “TDMA”) type mobile communication system, which includes: a plurality of radio base stations whose interfaces with a control device are different type, such as a main digital cordless phone, a PHS (Personal Handy-phone System) radio base station (for example, a general type radio base station (i.e., a radio base station connected with an ISDN line), an IP (internet protocol) connection type radio base station (i.e., a radio base station connected with the internet protocol network); a mobile phone connected wirelessly with those radio base stations; and an exchange (e.g., a private branch exchange) wired with those radio base stations, wherein frame synchronization in a radio section between the radio base stations is established.
As referred to Japanese Patent Laid-open Publication No. 2001-145155, column Nos. [0001] to [0004], for example, there is a TDMA type mobile communication system includes: a general type radio base station; a main digital cordless phone; a private branch exchange equipped with a control device which controls the general type radio base station and the main digital cordless phone; a mobile phone connected wirelessly with the general type radio base station; and the digital cordless handset connected wirelessly with the main digital cordless phone, wherein the mobile phone can be communicatively connected through the general type radio base station, and the digital cordless handset can be communicatively connected through the main digital cordless phone.
FIG. 16 is a block diagram showing a schematic configuration of the inside of the general TDMA type mobile communication system.
The TDMA type mobile communication system 200 shown in FIG. 16 is arranged for business in the private area of an office or a factory, and includes: a plurality of general type radio base stations 1; a plurality of business main digital cordless phones 2; a private branch exchange 4 communicatively connected with the general type radio base stations 1 through an ISDN line 3 and communicatively connected with the main digital cordless phones 2, the private branch exchange 4 having a control device; a mobile phone 5A wirelessly connected with the general type radio base stations 1; and a digital cordless handset (as will be simply called the “mobile phone”) 5B wirelessly connected with the main digital cordless phone 2.
The general type radio base station 1 executes calling control procedure in a ratio section on the basis of the RCR STD-28 as the standards of Association of Radio Industries and Business (ARIB), whereby services such as voice communications and data communications can be provided for the mobile phone 5A.
The main digital cordless phone 2 takes a one-to-one direct communication with the mobile phone 5B. Accordingly, electric waves are transmitted at the self operation timing without synchronization with another general type radio base stations 1, whereby services such as voice communications can be provided for the mobile phone 5.
The RCR standards regulate the frequency bands for voice communications between the mobile phone 5A and the general type radio base station 1 and the information configuration of the control CH signal as the call controlling signal which is used for frame synchronization and a carrier number. However, in same type mobile communication systems, common two frequencies, namely 12 ch and 18 ch are allocated to the control CH signal for the business, one of the same type mobile communication systems is obliged to avoid electric wave interference from another of the same type mobile communication systems.
When the general radio base stations 1 (1A, 1B) in the same system transmit the electric waves at an arbitrary timing, and their electric waves interfere with each other, electric wave using efficiency is reduced. In order to prevent the reduction of the electric wave using efficiency, therefore, it is necessary to match between electric wave transmission timings of the TDMA-TDD frames of the general type radio base stations 1 (1A, 1B) to retain frame synchronization between those general type radio base stations 1 (1A, 1B).
FIG. 17 is an explanatory diagram showing the concept of the TDMA-TDD frame which is used at a radio section of the TDMA type mobile communication system.
A TDMA-TDD frame 300 shown in FIG. 17 has a slot configuration of a radio section of second generation cordless telephone system standards, which is standardized by the ARIB. One slot 310, which is minimum unit, is composed of 240 bits (625 μs), and eight slots make one frame (5 ms).
Moreover, the one slot 310 is composed of a transient responding ramp time 311 of 4 bits, data 312 such as control information of 220 bits and a guard bit 313 of 16 bits. Between the succeeding slots 310, the total 20 bits of the guard bits (of 16 bits) 313 and the transient responding ramp time (of 4 bits) 311, that is, 52.1 μs make the allowance which absorbs the deviation between the slots.
In case the general type radio base stations 1 (1A, 1B) are in a frame synchronous state, as shown in FIG. 17, the electric waves for the TDMA-TDD frames of the general type radio base station 1A and the general type radio base station 1B are transmitted at the common timing. In order to hold this synchronized state, the deviation between the TDMA-TDD frame of the general type radio base station 1A and the TDMA-TDD frame of the general type radio base station 1B has to be confined in the allowable range.
Here, the operations till the general type radio base stations 1 retain the frame synchronization with each other will be described.
Each general type radio base station 1 establishes the frame synchronization between the general type radio base stations 1 (1A, 1B) by receiving synchronous signals for the reference of the frame synchronization through the ISDN line 3 of the private branch exchange 4 and by matching transmission timing of the TDMA-TDD frame with the synchronous signals thus received.
According to the TDMA type mobile communication system 200, moreover, when the frame synchronization is established between the general type radio base stations 1 (1A, 1B), it is necessary to determine transmitting timing of the control CH signal. The transmission position of the control CH signal has to be so controlled as to avoid the overlap between the general type radio base stations 1 (1A, 1B).
Here, a control CH signal transmission position segregation operation for preventing the overlap of the transmission positions of the control CH signal between the general type radio base stations 1 will be described. FIG. 18 is an explanatory diagram showing the general control CH transmission period conceptually.
A control CH transmission period 301 shown in FIG. 18 corresponds to an N-number (25≧N≧60) of frames 300 of 5 ms. Specifically, each general type radio base station 1 transmits the control CH signal of own station at an arbitrary frame position 1 to N and at a interval of N frames 300.
In case the general type radio base station 1A transmits the control CH signal at the timing “1”, for example, the transmission timing of the next control CH signal is at “N+1”. In other words, the general type radio base station 1 transmits the control CH signal of own station once at every interval of N frames 300.
Each of the general type radio base stations 1 use the control CH signal of own station to notify the transmission position of the control CH signal of own station within the interval range, in order of completion of starting such as the power ON. In response to this notified information, the surrounding general type radio base station 1 set up later determines the transmission position of the control CH signal of own station within the interval range while avoiding the transmission position of the control CH signal used by another general type radio base station 1, so that the transmission position of the control CH signal may not overlap between the general type radio base stations 1.
According to the TDMA type mobile communication system 200 thus far described, the synchronous signal is distributed from the private branch exchange 4 through the ISDN line 3 to each of the general type radio base stations 1. By matching the transmission timing of the TDMA-TDD frame of own station with the synchronous signal, therefore, the frame synchronization can be established between the general radio base stations 1 under the control of the same system.
According to the aforementioned TDMA type mobile communication system 200, moreover, when the frame synchronization between the general type radio base stations 1 is established, the transmission position of the control CH signal used by own station is searched within the interval range to determine the transmission position of the control CH signal of own station, so as to do not overlap the transmission position of the control CH signal of another general type radio base station 1. Then, the service operation is started.
According to the aforementioned TDMA type mobile communication system 200, moreover, the one-to-one direct communication is adopted between the main digital cordless phone 2 and the mobile phone 5. Therefore, the electric waves are transmitted at the self operation timing without any synchronization with another general type radio base station 1.